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Permit guides

ADU Permits in San Carlos: a Homeowner's Guide

San Carlos replaced design-committee review with objective standards — which makes the path clearer, but the tiers matter.

Last updated October 2026. Rules change; always confirm current requirements with the city before you build.

Who handles your permit

ADU permits in San Carlos run through the Community Development Department: the Building Division at 600 Elm St., San Carlos, CA 94070 (650-802-4261) and the Planning Division (650-802-4263, walk-in hours Monday–Friday 9am–12pm). Since December 2023, the city reviews single-family projects — including ADUs — against adopted Objective Design Standards, and the old Residential Design Review Committee was discontinued.

What you can build

Three review tiers

Where your project lands depends on size. Floor-area additions under 400 square feet, with a resulting total of 3,000 square feet or less (including garage), go straight to the Building Division — no neighborhood notification. Larger additions get Compliance Review by Planning, and the biggest tier adds neighborhood notification. Knowing your tier before you draw determines how much process you're signing up for.

What goes into the submittal

San Carlos also publishes unusually practical guidance for a common Peninsula situation — legalizing unpermitted work:

• First check the permit records: City of San Carlos at 650-802-4261 and the San Mateo County Assessor at 650-363-4500
• Legalization follows the same steps as a new construction permit — minimum site plan, floor plan, and construction details at 1/4" scale
• Work covered without inspections must be exposed for inspection — plan for opening walls, not just paperwork
• Consider whether reverting is simpler: a converted garage can sometimes just become a garage again, or an enclosed patio can conform as a patio

Where applications usually get stuck

• Wrong tier assumptions. Designing for the Building-only track and landing in Compliance Review with neighborhood notification adds weeks and a public-facing step.
• Unpermitted-work exposure. Owners underestimate how much finished work has to come open for inspection during legalization.
• Floor-area math in RS-6. The 800 sq ft statewide exemption interacts with the district's maximum floor area rules — get the calculation confirmed early.